The invention pertains to a catalyst for coating anodes, in electrochemical cells using a mixture of electronically conductive platinum metal oxides comprising RuO.sub.2, IrO.sub.2 and SnO.sub.2, and a process for its preparation, comprising forming a solution of water containing H.sub.2 IrCl.sub.6.mH.sub.2 O, RuCl.sub.3.nH.sub.2 O and SnCl.sub.2, and evaporating the solution to obtain a powder, which is dried and ignited at high temperatures prior to cooling.
It is known to accelerate and to promote electrochemical processes by catalysts applied to the electrodes. For this purpose, platinum metals and platinum metal oxides as well as mixtures thereof are preferably used on the anode side (for example the oxygen side of water electrolysis). In electrochemical cells which use a plastic polymer in the form of a diaphragm as a solid electrolyte, mixtures of RuO.sub.2 and IrO.sub.2 have proved particularly suitable. These platinum metal oxides are applied, as a rule in the form of powder, to the current collectors of the anode side (depassivated, porous titanium) (U.S. Pat. No. 4,326,943); Bockris, Conway, Yeager, White, Comprehensive treatise of electrochemistry, Vol. 2: Electrochemical processing, pages 61-78, Plenum Press, New York and London 1981). When operating such an electrochemical cell in practice, it was then found that even the electro-catalyst mixtures closest to the optimum suffer a certain amount of corrosion. As a result, the operating period and hence the life of the cell are limited. In accordance with the conditions of the surroundings (active oxygen, strongly acidic medium), a higher stability must be demanded from such a catalyst.
It has also been proposed in the past to add further oxides, for example those of the elements Ti, Sn, Bi, Sb and Ge, to the noble metal oxides in order, allegedly, to increase the stability of catalysts, another aim also being the formation of films (German Specification No. B-2,213,084; PCT Application No. WO 79/00,842). Nevertheless, it was not possible substantially to increase the life as compared with pure noble metal oxides as the catalyst mixtures. Moreover, the electrode activity suffered considerably in the case of coherent films.
There is therefore a need to search for further catalyst mixtures, a substantial improvement in the stability and a reduction in the material costs without a deterioration of the electrode activity appearing to be desirable.